1. The Field of the Invention
The present invention relates generally to apparatus and methods for purifying a gaseous mixture with a high H2(g) content so that it can be used safely in fuel cells and other applications without need for further processing. More particularly, the invention is directed to apparatus and methods for selectively oxidizing carbon monoxide in the presence of hydrogen gas, thereby allowing the purified gaseous mixture to be used to power fuel cells.
2. The Relevant Technology
Hydrogen gas can readily be produced by well-known processes such as partial oxidation of a hydrocarbon with air or oxygen, or steam reforming of a hydrocarbon. Historically the major producer of hydrogen gas has been the petrochemical industry, which has also been the major consumer. More recently, however, advances in fuel cell technology have prompted the development of technologies for the production of hydrogen gas suitable for use in fuel cells. These new technologies have included methods of producing and purifying hydrogen on a smaller scale; i.e., on a scale appropriate for fuel cell usage. It is of particular importance, of course, to purify the hydrogen gas of those impurities that adversely affect the performance of the fuel cell.
In fuel cells of the PEM type (polymer electrolyte membrane (PEM), or polymer electrolyte fuel cell (PEFC)), carbon monoxide readily poisons the anode catalyst, even at very low levels. While a carbon monoxide impurity of no greater than about 100 ppm can be tolerated, even a CO concentration of about 5 ppm can have a significant adverse effect on fuel cell performance. Thus, the removal of carbon monoxide from fuel cell gas mixtures down to very low levels (preferably less than about 5 ppm) is of particular importance for PEM fuel cell applications.
The CO content of a hydrogen-containing gas can be substantially reduced by passing the gas through a water gas shift reactor. This well-known purification method, however, is not suitable for removing trace amounts of CO from hydrogen gas mixtures. Further purification can be achieved by two well-known methods: passing the gas through a catalyst to reduce the CO impurity to methane, and selectively oxidizing the CO impurity to CO2.
Each of these known methods has disadvantages. The former method has the disadvantage that three moles of H2 gas are consumed for every mole of CO removed (CO+3H2xe2x86x92CH4+H2O). Moreover, if carbon dioxide is also present in the gas mixture, it may also be reduced to methane and water, at a cost of four moles of H2 gas per mole of carbon dioxide removed.
In selective oxidation as taught by the prior art, air (or oxygen) is added to the hydrogen gas mixture. The amount of air (or oxygen) added is equal to or greater than that which is stoichiometrically required for complete oxidation of the carbon monoxide impurity. The resulting mixture is then passed through a reactor containing a noble metal catalyst at an accurately controlled temperature to oxidize the carbon monoxide to carbon dioxide, while only a small portion of the hydrogen gas is oxidized.
For example, U.S. Pat. No. 3,216,783 discloses a process for the selective oxidation of carbon monoxide in hydrogen-rich gas mixtures by contacting the gas mixture with a supported platinum catalyst in the presence of oxygen. More recent work has focused on improving various aspects of the selective CO oxidation technology. Thus, U.S. Pat. No. 5,271,916 discloses a method of operating an H2xe2x80x94O2 fuel cell which includes, in part, catalytically oxidizing CO in a hydrogen-rich feed stream in two stages. The feed stream is first mixed with a predetermined quantity of oxygen, and the CO is oxidized at a first temperature on a first catalyst. The output stream is then mixed with a second predetermined quantity of oxygen, and the CO is oxidized at a second temperature on a second catalyst. The use of a dual-stage method reportedly produces a hydrogen-rich stream having less than 0.01% CO, without substantial reaction of the hydrogen.
Other references disclose methods of oxidizing CO using two air or oxygen streams (e.g., U.S. Pat. No. 5,432,021), two catalytic stages (e.g., U.S. Pat. Nos. 5,330,727 and 5,456,889), and various types of temperature control (e.g., U.S. Pat. Nos. 5,456,889 and 5,518,705) and/or oxygen-containing gas flow control (e.g., U.S. Pat. No. 5,637,415). Still other references attempt to optimize the CO oxidation selectivity and other factors using different catalyst systems, such as alumina-supported ruthenium or rhodium (Oh and Sinkevitch, Journal of Catalysis, 142, 254-262, (1993)); zeolite-supported platinum (Watanabe et. al., Applied Catalysis A: General, 159, 159-169 (1997) and Watanabe et. al., Chemistry Letters, 21-22 (1995)); gold supported on manganese oxides (Haruta et. al., Journal of Catalysis, 168, 125-127 (1997)); and copper supported on alumina-based mixed metal oxides (Eguchi et al., Applied Catalysis A: General, 169, 291-297 (1998)).
All of these references, however, are subject to important disadvantages inherent in conventional methods of catalyzed CO oxidation in hydrogen-rich gas mixtures. One of these disadvantages relates to the control of the temperature within the reactor. Accurate temperature control is necessary to promote complete and selective CO oxidation. If the temperature is too low, the oxidation is slow, and carbon monoxide can escape the reactor unoxidized. If the temperature is too high, the oxidation is rapid but unselective: too much of the added oxygen is consumed by reaction with the hydrogen gas, and again carbon monoxide can escape the reactor unoxidized. Although critical, accurate temperature control is difficult to achieve. The oxidation of carbon monoxide is strongly exothermic (xcex94H=xe2x88x9268.6kcal/mol CO), and the full heat of the CO oxidation reaction is liberated within the reactor, making control of the reaction temperature a matter of some difficulty.
A second disadvantage of these conventional methods is that they teach that air (or other oxygen-containing gas) is to be mixed with the hydrogen-rich gas, to provide the necessary oxygen for CO oxidation. While the amount of oxygen added is too small for the final mixture to be flammable, during the mixing process the mixture passes through a composition state which is not merely flammable but is potentially explosive. Although the amount of gaseous mixture in this hazardous condition may be relatively small when the system is operating normally, it may become much larger during a malfunction. Thus, use of these conventional methods raises serious safety concerns.
Thus, there is a need for a method which overcomes the disadvantages of the prior art while still allowing selective removal of carbon monoxide from hydrogen gas mixtures.
A principal object of the present invention is to improve the purification of gases having a high hydrogen gas content by providing a novel process and reactor system in which carbon monoxide can be selectively oxidized in the presence of hydrogen without significant oxidation of the hydrogen taking place.
A further object of the present invention is to minimize the problems of temperature control, which occur when carbon monoxide is oxidized, by performing the oxidation in such a manner that only a fraction of the heat of oxidation is released into the reactor.
Still another object of the present invention is to perform the selective oxidation of carbon monoxide without creating a potentially hazardous hydrogen-oxygen gas mixture.
To achieve the foregoing objects, and in accordance with the invention as embodied and broadly described herein, methods and apparatus have been developed for selectively oxidizing carbon monoxide in a gas mixture containing hydrogen gas without the necessity of mixing air into the gas. This is accomplished by alternately (1) contacting the gas mixture with an oxygen transfer catalyst in an oxidized state to oxidize the carbon monoxide in the gas mixture to carbon dioxide and reduce the oxygen transfer catalyst to a reduced state; and (2) reoxidizing the oxygen transfer catalyst by contacting it with an oxygen-containing gas, such as air. The method is conveniently carried out at a temperature at which the CO oxidation proceeds relatively rapidly, but the undesired oxidation of hydrogen gas proceeds relatively slowly, or not at all.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.